Wheat streak mosaic virus (WSMV), vectored by Wheat curl mite (WCM), causes disease of wheat plants of great economic importance in the USA and Canada. Recently, the virus has been identified in Australia. In a short span of time, the virus has spread to all major wheat growing areas. WSMV, a tritimovirus of the Family Potyviridae has a monopartite genome of single-stranded RNA (ssRNA) with messenger polarity and a genome size that varies from 9,339 to 9,384 nucleotides depending upon the isolate (Choi et al., 2001; Rabenstein et al., 2002; Stenger et al., 1998). The host range of WSMV is restricted to species in the family Gramineae and it is naturally transmitted by the wheat curl mite Aceria tosichella Keifer (Harvey and Seifers, 1991; Seifers et al., 1998; Slykhuis, 1955).
Wheat Streak Mosaic disease is one of the most destructive viral diseases of wheat (Conner et al., 1991; Jiang et al., 1993; Makkouk and Kumari, 1997; Nyitrai, 1991). For instance, in the Great Plains of North America endemics may cause yield losses up to 100% (French and Stenger, 2003; Stenger et al., 2002). In Australia, WSMV was first identified in 2003 in South Australia, Victoria, New South Wales and Queensland, followed by Western Australia in 2006 and more recently in Tasmania (Dwyer et al., 2007; Ellis et al., 2003; Ellis et al., 2004), with losses reaching 80% in some instances (Dwyer et al., 2007; Murray et al., 2007).
Two sources of natural resistance have been described to date in wheat and in its wild relatives but have proved to be temperature sensitive (Seifers et al., 1995; 2006) and of limited usefulness. One of those sources, a Thinopyrum intermedium chromosomal translocation to wheat, can incur a significant yield penalty in the absence of the virus (Baley et al., 2001; Divis et al., 2006; Sharp et al., 2002). The other resistance has been released once in the wheat cultivar RonL (Seifers et al., 2007). Synthetic resistance has been reported (Li et al., 2005) using a coat protein gene of WSMV. However, the resistance was only partial—the plants were not immune to the virus—and was unstable in later generations of the transgenic plants. Since all viruses included in the Potyviridae are thought to encode suppressors of gene silencing, the loss of transgene silencing and the observed unstable resistance may have been associated with suppression of silencing.
There is therefore a need to create improved sources of resistance to minimize losses, especially for environments with higher early season temperatures where the existing resistances break down.